A major performance characteristic of a coil spring is, as is known, its resistance to fatigue failure, such a resistance providing long working life under cyclic stresses. Fatigue life of springs is considerably shortened by scratches, notches, microcracks and other surface irregularities, this becoming even more pronounced in instances where scratches and cracks coincide in direction with the direction of working loads exerted on the springs. Increased fatigue life is normally attained by surface hardening through the employment of such widely varying techniques as flame treatment, surface impregnation, mechanical or thermomechanical hardening, etc.
At present hardening articles in the form of helical coils made of round wire with uniform degree of surface hardness circumferentially of the coil turn cross-section and low surface roughness poses considerable difficulties. There are known a number of methods of surface hardening springs by plastic deformation.
Widely used in the art is the shotblasting technique, wherein steel or cast iron pellets or balls are thrown at high velocity against the surface of a spring being processed. However, this method is inherently disadvantageous in that a spring thus treated is not uniform in surface hardness circumferentially of a turn thereof and has excessive surface roughness. In consequence, this causes the formation of minute stress concentrations on the surface which reduces the fatigue strength of the spring.
In another known method use is made of loose balls accommodated together with articles, such as screws and springs, in a closed volume and subjected to ultrasonic oscillations, the balls thereby effecting surface hardening of the articles.
However, this method fails to provide a uniform depth of the hardened layer on the surface of articles shaped as continuous helical coils.